I understand it's not the same beast as a single-rocket launch, but I'm sure they've already done much if not most of the theoretical portion of the design and computer simulation. They have a functional rocket design that they are reusing, so all that would be left is test launches to confirm they can get the three first stage rockets to function together and separate without causing issues in the second stage.
Ideally, the entire first stage would be recoverable, but in terms of mission success "all" they have to do is get the second stage to the proper position at the proper velocity, and then separate without issues. Anything beyond that is, technically, irrelevant to mission success.
Edit: From what I can tell, the "new" challenges to a Heavy launch are:
-Coordination between the three engines in the first stage (to my utterly untrained knowledge, this could be worked out with ground tests to confirm the three engines can coordinate and react to simulated requirements in changing thrusts)
-Structural integrity of the first stage (again, to my minimally trained knowledge, this is a matter of determining stresses on the rockets and designing the connectors to be sturdy enough to withstand said stresses + whatever safety margin. Obviously it's the unaccounted for that is problematic, but I'm unaware of any multi-rocket stages breaking loose from each other?)
-Booster rocket separation (given that the first stage rockets are all supposed to be landing, the real risk here is the boosters colliding with the centerline rocket. Failure to detach shouldn't pose a problem to mission success in terms of fuel requirements: the fuel that would have been used to land the stage should be adequate to ensure mission success at the cost of not landing the rockets, I would imagine)
I could be wrong, but the emergency escape mechanism has already passed the Falcon 9 requirements at the...I'm sorry, I've forgotten what it's called. The moment when the stresses on the rocket make an emergency escape the most likely to fail. With at least two Falcon Heavy launches planned for this year, I'd be surprised if they have to delay the planned trip by more than a year.